As the Atari 1200XL does not have a PBI port, it's expansion possibilities are limited to the SIO port and cartridge port. Since the cartridge port is in a "tunnel", on the left side, extension cards have been made that allow for the cartridge to be plugged in outside of the tunnel. As there is plenty of space, I decided to add a ROM and logic to the card.

This card can be plugged into the cartridge port, in the Atari 1200XL, and left in place. On the left are two switches. One switch enables the ROM on the card and the other switch enables the BASIC programming language on the card. Enabling the ROM will result in the normal operating system being replaced by the newest operating system available. (This is presently OS 6.42.) Two versions of BASIC are provided. The default version, of BASIC, is Altirra BASIC 1.59. Once the READY prompt is on the screen, the user can select Atari BASIC (Revision C) by pressing Control/F1.

OS 6.42 will provide the following upgrades: 1. Numerous bug fixes. 2. High speed SIO routines for use with high speed SIO drives. i.e. Happy 1050. 3. Control/Shift/Delete cold starts the computer 4. Faster keyboard response 5. Left margin is set to 0 6. Built-in CX-85 numeric keypad handler for joystick port 2. (Toggled via Control/Shift/3.) 7. Fast Math F+ floating point routines. Floating point math speed is roughly doubled. 8. Control/F4 toggles Fast Math F+ and LED 1 displays high speed when lit. 9. LED 2 lights up with SIO access. 10. WozMon is included and can be started with Control/Shift/Inverse 11. Cassette handler fix has been added to improve the reliability when reading cassettes. 12. Cassette baud rates of up to 900 baud are now supported. (i.e. Poke 600,90 to set 900 baud.) 13. Numerous other improvements as per this thread: https://forums.atariage.com/topic/347696-compiling-a-new-atari-os/

Additional OS Notes: 1. LEDs will not work if the computer has memory in excess of 64k. 2. The OS runs from RAM. As a result, some software will not be compatible with a RAM-based OS. (The front switch disables the OS on the card for just this purpose.)

The pictures show the card, the card installed, and the card with a cartridge. Only the card is for sale.